Chinese Journal of Chemistry, 2008, 26, 1215—1218 Full Paper

* E-mail: chemliup@whut.edu.cn; Tel.: 0086-027-63410197; Fax: 0086-027-87651773 Received October 7, 2007; revised February 18, 2008; accepted March 14, 2008. Project supported by the National Natural Science Foundation of China (No. 50702040) and Open Fund of State Key Laboratory of Advanced

Technology for Materials Synthesis and Processing of Wuhan University of Technology (No. WUT2006M01).

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Thermodynamic Studies of Electrostatic Self-assembly of Poly Diallyldimethylammonium Chloride on Proton

Exchange Membrane

LIU, Peng*,a(刘鹏) LI, Xia(李曦) PAN, Mub(潘牧) a Department of Chemistry, School of Sciences, Wuhan University of Technology, Wuhan, Hubei 430070, China

b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, Hubei 430070, China

The electrostatic self-assembly of polymer on proton exchange membrane was studied by calorimetric tech-nique. The titration of poly diallyldimethylammonium chloride (PDDA) into Nafion membrane was designed and performed to determine the thermodynamic parameters. The enthalpy change mrH∆ � and binding constant K in the process of self-assembly were obtained from data analysis with the help of Origin. According to the calculated thermodynamic parameters, the electrostatic self-assembly of PDDA on the proton exchange membrane is an “en-thalpy-driven” reaction. The released heat indicates decrease of energy, which is helpful for the occurrence of the self-assembly process, and the degree of disorder is reduced, which went against the adsorption process. As to every ion bond, the value of mrH∆ � of DDA is beyond PDDA because a small molecule can bind itself to the membrane without steric hindrance.

Keywords self-assembly, proton exchange membrane, titration, bind, steric hindrance

Introduction

Catalyst coated membrane was concerned about catalyst efficiency in the usage of proton exchange membrane fuel cells.1 In recent years, electrostatic self-assembly technology was applied to the fabrication of catalyst coated membrane in order to improve the water-gas-ion three-phase reaction zone on the surface of platinum catalyst.2 Poly diallyldimethylammonium chloride (PDDA), as a kind of charged polymer, was usually used to modify platinum nano-particles.3,4

A sulfonic group is usually chosen as a proton ex-change carrier of proton exchange membrane of fuel cells. In solution, the hydrophilic sulfonic group will extend into liquid phase, and an electric double layer

was formed on the solid-liquid surface. When PDDA with positive charge diffuses on the proton exchange membrane, it will be captured by a negatively charged sulfonic group with electrostatic force. Monomolecular layer of PDDA is then formed on the surface of proton exchange membrane, as shown in Figure 1.

Free anions can also interact with PDDA that has been absorbed on proton exchange membrane. No doubt, assembly-desorption of all cations will reach dynamic balance in some time. Theoretically, that a free cation was captured by the membrane surface with negative charges was accompanied with the formation of a quasi ion-bond, which is weaker than a normal ion bond in ionic crystal. In the process of formation of a quasi

Figure 1 Electrostatic self-assembly of PDDA on proton exchange membrane.

1216 Chin. J. Chem., 2008, Vol. 26, No. 7 LIU, LI & PAN

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ion-bond, the energy of the mixture system was reduced and the heat released. After electrostatic self-assembly, it is a key question whether PDDA can stably exist on the surface of the proton exchange membrane. Catalysis efficiency and service life of the catalyst coated mem-brane depend on the thermodynamic properties of bind-ing between PDDA and the proton exchange membrane, such as change of enthalpy, entropy, Gibbs free energy, and binding constant. In this study, the thermodynamic parameters in the process of electrostatic self-assembly were obtained by a microcalorimetry technique.

Materials and methods

Materials

Diallyldimethylammonium chloride (DDA) and PDDA (molecular weight≈5000, Figure 2) were ob-tained from Aldrich. Proton exchange membrane (Nafion 112) was obtained from Dupont. All solutions were prepared with doubly distilled water.

Figure 2 Diagram of molecular structure of PDDA.

Instrumental

A TAM Air instrument (Thermometric AB, Sweden), which is an 8-channel heat conduction microcalorimeter for measurements of heat flow, can monitor con- tinuously heat released or absorbed in a series of proc-esses. The performance and the details of this instru-ment have been described before.5,6 The instrument was designed to perform the titration experiment.7 With the buffer in a reference ampoule unchanged, the sample ampoule was installed in the titration setting, as shown in Figure 3. The titration setting contains a syringe, one reservoir, one vent and some tubules to connect them. When performing an experiment, the reference ampoule was filled with buffer that has the same heat capacity as the sample, the syringe and reservoir were filled with titrant, the titration ampoule was filled with the material to be titrated, and the calorimeter was equilibrated to the desired temperature.

Measurements

At 298.15 K, with the reference ampoule filled with buffer, the sample ampoule was filled with 5 mL of dis-tilled water containing 5 g of nafion membrane. The syringe and reservoir were filled with titrant—10－4

mol•L－1 PDDA. After the titration settings were put into the calorimeter, two hours were spent to equilibrate to the same temperature of the thermostat. The titration experiment was performed by injection of 0.1 mL of PDDA solution into the sample ampoule each time. The other group of titration experiment was performed by injection of DDA (0.02 mol•L－1) solution into the sam-ple ampoule.

Figure 3 Schematic isothermal titration device on TAM Air.

Results and discussion

Sulfonic groups on a proton exchange membrane can interact with molecules in solution by electrostatic force. At the same time, the bound molecules can also escape from the proton exchange membrane because of thermal motion. On the proton exchange membrane, one sulfo-nic group can absorb one site of positive charge. There-fore, a monomolecular layer will appear on the surface. When the absorbed molecules occupied all the surface of the proton exchange membrane, the surface was saturated and no molecule could be absorbed any more. Therefore, equilibrium will reach between absorption and desorption, as shown in the following equation.

P＋S� [PS]

where P is PDDA, S is sulfonic group, [PS] is com-plexes of PDDA with the sulfonic group. Cp is the con-centration of free PDDA, Ct is total comcentration of PDDA, Mt is the total molar number of sulfonic groups, Ms is the molar number of —SO3 that has not absorbed PDDA, Mps is the molar number of —SO3 that has ab-sorbed PDDA, k+ and k− are absorption constant and desorption constant, respectively.

The absorption rate: p sr k C M+ +＝ (1) The desorption rate: psr k M− −＝ (2) At equilibrium: r r+ −＝

(3)

K can be regarded as the binding constant of PDDA

with the sulfonic group.

ps

pst t ps( ) ( )

MK

MC M M

Vi

＝

－ －

(4)

t t

2t t t t

ps

( )

( ) 4

2

VC M

K

VC M V C M

KM

⎡ ⎤⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦

i i

＋ ＋ －

＋ ＋ －

＝

(5)

ps

p s

M kK

C M k+

−＝ ＝

Self-assembly Chin. J. Chem., 2008 Vol. 26 No. 7 1217

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 0.1V i＝ ＋ , 4t

0.110

5 0.1

iC

ii

－

＝＋

, tM ＝10－4 mol,

54

54 2 9

ps

10 5 0.1( 10 )5 0.1

10 5 0.1( 10 ) 4 105 0.1

2

i i

i K

i ii

i KM

⎡ ⎤⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦

－

－

－

－ －

＋＋ ＋ －

＋

＋＋ ＋ －

＋＝

×

(6)

where i is times of injection.

psQ H M∆ i＝

∆H is molar enthalpy change.

54

254 9

10 5 0.110

5 0.1

10 5 0.110 4 10

5 0.1

2

i i

i K

i ii

i KQ H

⎡ ⎤⎛ ⎞⎢ ⎥⎜ ⎟⎝ ⎠⎢ ⎥⎢ ⎥⎢ ⎥⎛ ⎞⎜ ⎟⎢ ⎥

⎢ ⎥⎝ ⎠⎣ ⎦∆ i

－

－

－

－ －

＋＋ ＋ －

＋

＋＋ ＋ － ×

＋＝

(7)

Analysis of the processed calorimetric curve was performed with the help of non-linear fitting of Micro cal Origin.8-10 The binding affinity (K) and binding en-thalpy ( mrH∆ � ) could be calculated from Eqs. (4), (5) and (6), which are shown in Table 1.

The Gibbs free energy mrG∆ � and entropy mrS∆ � of the binding reaction can be obtained from

mr lnG RT K∆ ＝－

� (8)

m m mr r rG H T S∆ ∆ ∆＝ －

� � � (9)

At 298.15 K, PDDA was titrated into solution. The thermogenic curve in titration process was shown in Figure 4. The proposed binders will show a concentra-tion dependence of the measured heat from the micro-calorimetric titration experiments, while a non-binder will essentially show only effects associated with heat of dilution and all peaks will be of the same size during repeated injections. After subtraction of dilution heat, the integrated heat of each injection was shown in Fig-ure 5, from which it could be seen that heat was pro-duced accompanying incremental additions of PDDA.

Both negative values of mrH∆ � and mrS∆ � were observed in the process of electrostatic self-assembly.

Figure 4 The thermal curves for PDDA being titrated into pro-ton exchange membrane.

Figure 5 The accumulated heat with incremental additions of PDDA.

The standard enthalpy mrH∆ � can be considered as an indicator of the increase of bond energies in the binding process, while the standard entropy mrS∆ � reflects the decrease of disorder of the system during the reaction process. In the process of electrostatic self-assembly, the cations migrated from the solution of three dimensions to the surface of silica colloid and were fixed by nega-tive charges with electrostatic force. The degree of dis-order inevitable reduced and the entropy of the system decreased, which went against the adsorption process. However, the exothermal change of electrostatic self-assembly was observed, which is helpful for the occurrence of the reaction. With the changes of enthalpy and entropy in this experiment having the opposed con-

Table 1 Thermodynamic parameters for the electrostatic self-assembly

T/K K mrH∆ � /(kJ•mol－1) mrG∆ � /(kJ•mol－1) mrS∆ � /(J•mol－1•K－1)

PDDA 7.89±0.52×1016 －125.38±10.25 －96.44 －97.07 298.15

DDA 2.50±0.16×108 －56.11±4.83 －47.93 －27.44

PDDA 6.22±0.31×1015 －108.46±9.76 －93.14 －49.62 308.15

DDA 7.89±0.23×107 －49.78±4.15 －46.58 －10.37

1218 Chin. J. Chem., 2008, Vol. 26, No. 7 LIU, LI & PAN

© 2008 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tribution, the electrostatic self-assembly of PDDA on the proton exchange membrane was considered as an “enthalpy-driven” reaction.11,12

Diallyldimethylammonium chloride (DDA), the monomer of PDDA, also has affinity for the proton ex-change membrane. The same experiment was performed on the electrostatic self-assembly of DDA on the proton exchange membrane. The thermodynamic parameters were obtained and shown in Table 1.

Under the same condition and with the same concen-tration of quaternary ammonium, more heat was re-leased in the process of self-assembly of DDA than in that of PDDA. The results indicate that interaction be-tween the monomer and membrane is stronger than that between the polymer and membrane, and bond energies are increased to some extent. During the process of self-assembly, small molecules move more easily and can reach the surface of the proton exchange membrane more quickly. As to the polymer, the molecular chains were folded in three dimensional space, which brought difficulties in moving, approaching and binding on the proton exchange membrane. Besides, steric hindrance weakens the binding force between PDDA and the sul-fonic group on the membrane. Of course, for one mole-cule, PDDA has stronger force with membrane than DDA and more change of thermodynamic parameters were observed because every polymer chain is com-posed of about 39 monomers and contains 39 positive charges.

The surface charge of the membrane is sensitive to the nature and concentration of counter-ions. In the process of adsorption, because of electrostatic attraction, the surface charge of the membrane will decrease. Thus, it is of interest to investigate the surface phenomena of the membrane in the presence of different counter-ions in solutions. From the point of thermodynamics, all the chemical, physical, biological processes are accompa-nied by the change of thermodynamic parameters. De-termination of the thermodynamic parameters is impor-tant because the enthalphy is a representive of energy of electrostatic self-assembly, while the entropy is con-nected with the extent of disorder. The two properties of

a system determine whether a reaction can take place according to the equation r r rG H T S∆ ∆ ∆＝ － , because the change in the Gibbs free energy rG∆ alone pro-vides a criterion for the spontaneity of a process at con-tant pressure and temperature. In recent years, micro-calorimetry has been applied to the study of binding reactions more and more widely. In many such studies, microcalorimetry was used to determine the thermody-namic parameters in solution reactions.13-15 This study provided with a method to obtain the thermodynamic parameters of self-assembly reaction, which is a hot spot of research.

References

1 Haile, S. M. Acta Mater. 2003, 51, 5981. 2 Wang, S. L.; Tang, H. L.; Pan, M.; Mu, S. C.; Yuan, R. Z.

Mater. Rev. 2003, 17, 37. 3 Antolini, E. Mater. Chem. Phys. 2003, 78, 563. 4 Shim, J.; Yoo, D. Y.; Lee, J. S. Electrochim. Acta 2000, 45,

1943. 5 Liu, P.; Liu, Y.; Chen, Y. G.; Qu, S. S. Chemistry 2002, (10),

682 (in Chinese). 6 Qin, C. Q.; Zhou, B.; Zeng, L. T.; Zhang, Z. H.; Liu, Y.; Du,

Y. M.; Xiao, L. Food Chem. 2004, 84, 107. 7 Liu, P.; Li, X.; Pan, M. Acta Phys.-Chim. Sin. 2007, 24, 161. 8 Freire, E.; Mayorag, O. L.; Straume, M. Anal.Chem. 1990,

62, 950. 9 Liang, Y.; Du, F.; Sanglier, S.; Zhou, B. R.; Xia, Y. J. Biol.

Chem. 2003, 278, 30098. 10 Jiang, N.; Li, P. X.; Wang, Y. L.; Wang, J. B.; Yan, H.;

Robert, K. T. J. Phys. Chem. B 2004, 108, 15385. 11 Liu, P.; Liu, Y.; Xie, Z. X.; Shen, P.; Qu, S. S. Chin. J.

Chem. 2003, 21, 693. 12 Paola, G.; Valeria, F.; Gilli, G.; Borea, P. A. J. Phys. Chem.

1994, 98, 1515. 13 Guo, M.; Lu, W. J.; Yi, P. G.; Yu, Q. S. J. Chem. Thermo-

dyn. 2007, 39, 337. 14 Privalov, P. L. Biophys. Chem. 2007, 126, 13. 15 Laugel, N.; Betscha, C.; Winterhalter, M.; Voegel, J. C.;

Schaaf, P.; Ball, V. J. Phys. Chem. B 2006, 110, 19443.

(E0710072 CHENG, F.)